Hypothesis: Sex-Related Differences in ACE2 Activity May Contribute to Higher Mortality in Men Versus Women With COVID-19

Abstract

Angiotensin-converting enzyme 2 (ACE2) facilitates the cellular entry of the severe acute respiratory syndrome-associated coronavirus 2 (SARS-CoV-2), which causes the coronavirus-2019 (COVID-19) disease. Recent reports have shown worse outcomes in men with COVID-19 infection compared to women. We review the hypothesis that sex-related differences in outcomes in COVID-19 are due to different activity of ACE2 between men and women. We also show that studies in humans have demonstrated no significant difference in serum ACE2 levels between healthy men and women. However, men with hypertension and heart failure typically have higher level of serum ACE2 activity compared to women. We hypothesize that the worse outcomes in men with COVID-19 compared to women is likely due to higher prevalence of hypertension and heart failure among men compared to women. To test this hypothesis, studies to compare the outcomes of COVID-19 infection between men and women with no preexisting heart diseases are needed.

Introduction

Angiotensin-converting enzyme 2 (ACE2) is a zinc metallopeptidase, type I integral membrane glycoprotein, and one of the components of the renin-angiotensin-aldosterone system (RAAS). ¹ ACE2 is highly expressed in the testis, kidneys, heart, thyroid gland, adipose tissue, and oral mucosa, and expressed at lower levels in the lungs, liver, small intestine, colon, bladder, prostate, adrenal glands, and brain. ^1,2 ACE2 functions exclusively as a carboxypeptidase; it hydrolyzes its substrates by removing a single amino acid from their respective C-termini. Therefore, ACE2 has the ability to convert the decapeptide angiotensin I (Ang I) to Ang (1-9) and the octapeptide Ang II to Ang (1-7). Additionally, ACE2 has several other substrates, such as apelin-13, neurotensin- (1–11), dynorphin A-(1–13), β-casomorphin-(1–7), ghrelin 22, des-Arg ⁹ -[bradykinin]. ¹

ACE2 is encoded by the ACE2 gene, which is located on the X chromosome (location: Xp22.2; nucleotides 15 494 402–15 602 148, GRCh38.hg38 version). The clinical importance of this location has not been studied yet.

ADAM Metallopeptidase Domain 17 (ADAM17), also called tumor necrosis factor-α-converting enzyme (TACE), and transmembrane protease serine 2 (TMPRSS2) catalyzes ACE2 shedding from tissue into circulation. ³ This process is likely mediated by an effect of Ang II on Ang II type 1 receptors (AT1 R) ⁴ (Figure 1).

Figure 1.

ACE2 is a type I integral membrane glycoprotein. ADMA17 catalyzes ACE2 shedding from tissue into circulation; this function is mediated by Ang II via an action on AT1 R. Heart failure and hypertension are typically associated with higher levels of Ang II, which would subsequently lead to higher ACE2 shedding into circulation.

ACE2 has been identified as a receptor for the severe acute respiratory syndrome-associated coronavirus (SARS-CoV) and the SARS-CoV-2 ⁵ ; the latter causes the coronavirus-2019 (COVID-19) disease. The SARS-CoV-2 uses ACE2 as a cellular entry receptors to enter ACE2-expressing cells. ⁵ The cellular entry of SARS-CoV-2 into cells is facilitated by an interaction between the viral spike proteins and the extracellular domains of the transmembrane ACE2 proteins. This is typically followed by downregulation of the surface ACE2 expression. Due to the role of ACE2 as a receptor for SARS-CoV-2 that facilitates its cellular entry, it is been hypothesized that high levels of circulatory ACE2 would be associated with worse outcomes in patients with COVID-19 infection. Recent reports have shown that men generally experience a more severe form of COVID-19 infection and higher mortality rate compared to women. ^{6 -9} Rapp et al ⁸ showed that males with COVID-19 infection are at a higher risk of mortality compared to females with COVID-19 infection, and males are more likely to present with sepsis and hypoxia on admission. Ko et al ⁷ showed that males with COVID-19 infection are at higher risk of hospitalizations compared to females. It is possible that the sex-related differences in outcomes in COVID-19 are, in part, due to different tissue expression and serum activity of ACE2 between men and women (Figure 2). However, our understanding of the sex-related differences in ACE2 expression in tissues and its levels in plasma is extremely limited, and most of it is based on animal models. Herein, we summarize our current understanding of the sex-specific differences in ACE2 expression, particularly in the light of COVID-19 infection.

Figure 2.

The relationship between ACE2 expression and serum ACE2 activity is not well understood. The entry of SARS-CoV-2 to host cells is facilitated by an interaction between the viral spike proteins and the extracellular domains of the transmembrane ACE2 receptor. Higher levels of serum ACE2 was observed in men with heart failure and hypertension compared to women, and higher expression of ACE2 was observed in men type II pneumocytes compared to women. The higher level of ACE2 serum activity and tissue expression might explain, in part, the worse outcomes in men with COVID-19 infection (A) compared to women with COVID-19 infection (B).

Sex-Associated Differences in ACE2 Tissue Expression

Preclinical animal models of hypertension and obesity have shown conflicting results regarding ACE2 expression in males and females. Xie et al ¹⁰ found no significant difference in ACE2 expression in the lung between young male and female rats or middle-aged male and female rats. Both sexes had a significant age-associated decrease of ACE2 expression in lungs; however, the age-related decline in ACE2 expression was more remarkable in male rats than female rats. Liu et al ¹¹ described higher level of renal ACE2 activity in male mice than in female mice; however, there was no sex differences in ACE2 activity in the heart and lung. Dalpiaz et al ¹² observed higher level of cardiac ACE2 activity in male spontaneously hypertensive rats (SHR) than in female SHR. In another study in congenic hypertensive mRen(2).Lewis rats, ¹³ Pendergrass et al described higher renal cortical ACE2 activity in male rats compared to female rats. On the other hand, Pederson et al ¹⁴ demonstrated a higher ACE2 expression in pancreatic islets in female db/db mice than in males. ¹⁵ Milsted et al ¹⁶ studied the effect of Sry genes from the Y chromosome on the RAAS in a Chinese hamster ovary cells model and found that Sry1, Sry2, and Sry3 up-regulated the activity of the promoters of angiotensinogen, renin and ACE genes and down-regulated ACE2 promoter activity. While there is some discrepancy in the results, these preclinical studies suggest sex-related differences in ACE2 expression and activity.

There is paucity of studies that examined the difference in tissue ACE2 expression in men and women. Niu et al ¹⁷ studied the frequency of the 1075G allele and the 8790A allele, both of which are intronic polymorphisms of the ACE2 gene, in a Chinese population. In this study, the frequency of the 1075G allele was 30% in males compared to 23% in females, whereas the frequency of the 8790A allele was 36% in males compared to 32% in females. In their study, the 1075G allele was independently associated with an increase in the susceptibility to hypertension in men only. This observed difference could be because the ACE2 gene is located on the X chromosome, and in females, most genes on one X chromosome are typically inactivated. In a recent study, men had greater ACE2 expression in type II pneumocytes compared to women. ¹⁸

Sex-Associated Differences in Circulating ACE2 Activity

The relationship between circulating ACE2 activity and tissue ACE2 expression is not well understood. Studies have shown sex-associated differences in circulating ACE2 activity. One of the early studies that evaluated this difference was in kidney transplant patients. This study showed ACE2 activity to be higher in men compared to women among this population. ¹⁹ In a more recent study, ¹⁵ hypertensive men without heart failure had a statistically significant higher serum ACE2 activity compared to hypertensive women without heart failure; a similar sex difference was observed among patients with heart failure with reduced ejection fraction (HFrEF). Although healthy men and men with heart failure with preserved ejection fraction (HFpEF) tended to have higher serum ACE2 activity compared to matched women, this association was not statistically significant. In another study in healthy individuals, ²⁰ there was no sex-related difference in serum ACE2 activity. However, a significantly higher ACE2 serum activity was observed in women older than 55 years old compared to women younger than 55 years with no significant difference in serum ACE2 activity between older and younger men. Bernardi et al ²¹ also found no difference in ACE2 serum level between healthy men and women. In a recently published study in patients with heart failure, defined as left ventricular ejection fraction of ≤40% or plasma concentrations of B-type natriuretic peptide and/or N -terminal pro-brain natriuretic peptide >400 pg/mL or >2000 pg/ml, respectively, serum level of ACE2 was higher in men than in women. ²²

Effect of Sex Hormones on ACE2

In a SHR model, ¹² Dalpiaz et al observed that orchiectomy decreased the activity of ACE2, while ovariectomy increased its activity. In an MF1 mice model, Liu et al ¹¹ observed no effect of gonadectomy or the sex chromosomes on renal ACE2 activity in males; however, gonadectomized females had higher renal ACE2 activity than the intact females. A 1-month treatment with 17β-estradiol (E2) in gonadectomized mice reduced renal ACE2 activity regardless of the sex. ¹¹ On the other hand, E2 treatment of human atrial tissues resulted in a shift from ACE to ACE2 expression. ²³ In a study on normal human bronchial epithelial cells, ²⁴ E2 treatment down-regulated ACE2 expression. These data suggest that sex hormones modulate ACE2. This effect is, however, dependent on tissue type with significant difference between animals and humans.

Conclusion

In summary, studies in humans have generally shown no significant difference in serum ACE2 levels between healthy men and women. However, hypertension and heart failure are likely associated with higher level of serum ACE2 activity in men compared to women. This difference could indicate a sex hormone-dependent greater ACE2 expression in men compared to women. Our understanding of ACE2 tissue expression in humans is extremely limited, let alone our understanding of the sex-related differences of tissue ACE2 expression.

Men have higher prevalence of hypertension and HFrEF compared to women. The purported higher ACE2 activity in men, particularly in those with hypertension and HF, might explain, in part, the worse outcome in men with COVID-19 infection compared to women with COVID-19 infection. The location of the ACE2 gene on X chromosome could play a role in the higher ACE2 expression in men compared with women. However, this hypothesis remains a conjecture at the moment and needs to be explored. We recognize that there is no information on whether other factors such as patient demographics and socio-economic status and environment may affect the proposed sex-related differences in outcomes. These factors should be looked at in future studies.